The proposed work aims to test the hypothesis that the abrogation of negative regulatory pathways is central to the mechanism whereby hepatitis B virus (HBV) encoded X antigen ( HBXAg) causes hepatocellular carcinoma (HCC). This hypothesis is premised on the finding that HBxAg binds to and apparently inactivates a senescence factor (p55sen) isolated from a yeast two-hybrid screen, that it apparently suppresses the expression of another cellular gene with considerable homology to another senescence factor (p45SRF) using a PCR select cDNA subtraction approach, and that HBXAg appears to transcriptionally suppress the known senescence factor p21 WAF1ISDI1/CIP1. In order to evaluate the role of these senescence factors in hepatocarcinogenesis a series of HBXAg mutants will be tested for their ability to bind p55sen and/or down regulate p45 SRF or p21WAF1. Those mutants which do not bind p55 sen or fail to down regulate 45SRF or p21WAF1, will then be screened in a number of cell based assays which score for properties relevant to hepatocellular transformation. Preliminary work by the principal investigator has shown that HBxAg expressed in liver cell lines increases the resistance of these cell lines to anti-Fas and TGFb1 mediated apoptosis promotes cell survival in seru free medium, and stimulates growth in culture, in soft agar, and in nude mice. Hence, the proposed work in the first specific aim intends to determine whether HBXAg mutants which fail to bind to or down regulate one or more of these senescence factors correlate with the loss in one or more of these cellular phenotypes. The experiments aim to identify the importance of senescence factor inactivation to hepatocellular transformation mediated by HBxAg, which will contribute crucially to the understanding of HBV associated multistep hepatocarcinogenesis. The second specific aim aims to identify additional components of the p55sen and p45SRF pathways and the mechanism which mediates negative growth regulation. Other experiments are planned to assess whether HBXAg own regulation of p21WAF1 correlates with inactivation of the retinoblastoma gene product.